"Oh, it's you...."
"It's been a LONG time. How have you been? I've been really busy being dead. You know...after you MURDERED me? .... Okay look, we both said a lot of things that you are going to regret. But I think we should put our differences behind us. For science. You MONSTER."
These are the first words you'll hear out of GLaDOS when you and your new personality core friend Wheatly unknowingly re-awaken her after a few hundred years of... well.... being dead.
After reading GI's main article on Valve's upcoming revisit to the Aperture Science building, I took it upon myself to dig around and blow the dust of the cover of my copy of The Orange Box.
I replayed Portal, and like my first time, enjoyed every level.
But you find something different this time. The ending.
On March 3rd, 2010, Valve introduced (without any notice or press) an update to Portal, changing the ending and adding a few new achievements. It changes it slightly, but enough to allow an easy bridge to a sequel.
I urge you to replay it and find the different ending for yourself. However, if you want to avoid it, just send me a PM and I'll give you the lowdown on the different ending.

But here's and idea. Once you avert GLaDOS trying to dump you in an incinerator in the first Portal you'll here her chide:
"We are pleased that you made it though the final challenge where we pretended we were going to murder you. We are very, very happy for your success. We are throwing a party in honor of your tremendous success. Place the device on the ground, then lie on your stomach with your arms at you sides. A "party associate" will arrive shortly to collect you for your party. Make no further attempt to leave the testing area. Assume the party escort submission position or you will miss the party."
Obviously you ignore her going on to rip spheres from her breast and slowly murder her. This leads to the different ending. And the new introduction of a "party associate" leaves some things to ponder as well.
If you wish to watch the new ending with out revisiting the first Potal, the link below will take you to a subtitled HD Youtube video of it.
Portal 1 alternate ending
Speaking of those spheres you incinerated from GLaDOS.... Those are Personality Spheres. And if you'll recall some few hundred of them were awakened at the end of Portal 1 surrounding the fabled "cake."
These Personality Spheres have been since wondering the derelict establishment for hundreds of years, until one named Wheatley takes it upon himself to awaken Chell only to help him move from one rail to the other. As they have limited mobility, restricted to a few overhanging rails. He promises that if Chell can catch him and move him then he will show her the way out.
Well, Chell fails and drops Wheatley. Quick to forgive, the Sphere decides to be helpful anyways and the new duo go on to move about and find their way through the facility. All the while Wheatley, now comfortably situated in Chell's hands, is blabbering incessantly, proving to me more of a help than annoying.
Chell can plug him into various terminals where the chatty Sphere can work his magic. And this is extremely important in the facility at this point, because it is severely damaged and even overgrown in places.
Finally they come to a large cylindrical room with a few hundred breaker switches. Here Wheatley warns that only the switch to power the escape pod should be flipped - or else. Chell obviously can't spot the switch out of myriad of them, and so she plugs him in to do the dirty work for her. Almost immediately the ancient equipment fizzles and pops and the floor underneath them starts to move up. Wheatley scrambles mindlessly for an abort password, "AAAAAA? No, that's not it. AAAAAB? No? AAAAAC? You've got to be joking!"
Having failed, Chell and Wheatley find themselves in an all too familiar room.
GLaDOS creaks to life amid a flurry of sparks.
And the tests begin again. For science.
This time in Portal 2, things are different. This is a FULL GAME ladies and gents. This time you will find yourself pulling your hair out even more, with things like vacuum tubes, aerial faith plates, and companion cubes with reflective sides to reflect any unwanted lasers or redirect them as necessary. Valve assures that no one will be disappointed on the new things to come.
What's that? You say Co-Op would be cool? Well you're right. And they are accommodating that as well.
There is going to be a single player campaign as well as a Co-Op campaign (which can be done online, or split screen). However the Co-Op follows two bi-pedal robots, not humans. Made from a personality core and a turret. You and your buddy using these bots is where the REAL thinking will come in.
Oh and the robots like to hold hands too.

No.. really. They do.
With it's augmented length, fresh puzzle elements, polished presentation, and new cooperative campaign, Portal 2 will not be coming in any value pack. It will be right at home as a full, stand alone game. And rightfully so.
Due out this fall you won't have to wait long to rekindle that long lost love you so desperately wanted back from GLaDOS:
"We're a lot alike, you and I. You tested me. I tested you. You killed me, I...oh, no, wait. I guess I haven't killed you yet. Well. Food for thought during this next test."
She can't wait either.